SMS reaches a phone through a mobile messaging service without requiring a separate messaging app. One network message can carry 160 basic Latin letters or digits. Other characters can reduce that capacity. Device support, service availability and carrier routing affect whether a message can arrive.
The recipient uses the phone’s messaging application to read the text. The message includes a sender identity and can contain links.
Why 160 characters?
SMS messages have a fixed-size payload. With the GSM-7 character set, that payload holds 160 encoding units. An ordinary Latin letter such as A, or a digit such as 1, uses one unit. Extension characters such as € and ^ use two units each.
Longer messages are split into segments and reassembled on the receiving phone. Each segment reserves space for reassembly information, leaving room for typically 153 GSM-7 units. A message with 161 plain letters needs two segments. The first carries 153 letters. The second carries eight.
Text that cannot use GSM-7 is encoded as Unicode, often labelled UCS-2. A single segment holds 70 UTF-16 units. The character 漢 uses one unit. The emoji 😀 uses two. Longer messages carry typically 67 units per segment because reassembly information occupies some capacity. A visible emoji can combine several characters and use more units.
Carriers bill per segment. One character that changes the encoding can turn a one-segment message into several segments and increase its cost.
How is SMS different from MMS, RCS, and OTT apps?
The channels differ in the content they carry and the service the recipient needs.
| Reach | Media | Transport | |
|---|---|---|---|
| SMS | SMS-capable mobile service | Text and text links | Mobile messaging service |
| MMS | Supported carrier and device | Images, audio, video | Carrier messaging service |
| RCS | Supported device, service and business route | Rich media, buttons, receipts | IP |
| OTT apps (WhatsApp, etc.) | App users only | Rich media | Internet |
MMS adds media to a text conversation. Supported media include images, audio and video. Carriers transport these messages between supported services and devices.
RCS for Business uses phone numbers to address recipients. It adds read receipts, typing indicators and verified business branding. WhatsApp and other messaging apps use the internet. Recipients need the relevant app and account.
Google recommends configuring another channel, such as SMS, when recipients cannot receive RCS. Confirm RCS non-delivery before trying SMS. An uncertain response can otherwise lead to duplicate messages.
set up a fallback method of communication with another channel, such as SMS.
How does business SMS (A2P) work?
An application sends an A2P message to a person through a messaging provider. Verification codes and shipping updates use this path.
Your messaging provider connects the application to the recipient’s carrier. The send uses an eligible number or alphanumeric sender identity. Carriers apply sender, permission and volume requirements to business traffic.
What is SMS used for?
SMS carries one-time codes, transactional updates and marketing messages:
- One-time passcodes. SMS can carry a one-time code as part of a verification flow. Bird Verify manages code generation and delivery.
- Transactional alerts. These include order confirmations, shipping updates, appointment reminders and account notifications.
- Marketing. Promotions and campaigns sent to recipients who have opted in. In SMS campaigns, the audience permission, sender and content need to match the programme.
What about deliverability and compliance?
Carrier filtering, sender registration, recipient eligibility and network conditions can affect the outcome:
- Opt-in. Consent requirements depend on the message purpose and destination. Your sending workflow must honour opt-outs immediately so recipients can stop messages they no longer want.
- Sender identity. The number or sender ID you send from affects throughput, trust, and where you can send. Different identities suit different volumes and regions.
- Local registration regimes. Some markets require you to register your business and messaging programme before sending A2P traffic. For US local numbers, 10DLC registration connects your brand, campaign and numbers. Carriers use that registration when applying throughput and filtering rules. Sender requirements vary by destination.
Meeting the sending requirements does not guarantee delivery. Message records and carrier receipts describe the reported outcome.
FAQ
Does SMS support links and images?
SMS supports links as plain text, which the recipient's phone can make tappable. SMS does not carry images. MMS, RCS and messaging apps support media through their respective services.
How long can an SMS actually be?
One segment holds up to 160 basic GSM-7 characters, such as Latin letters and digits. Extension characters such as € count as two. A single Unicode segment holds 70 UTF-16 units. The character 漢 uses one unit. The emoji 😀 uses two. Longer messages are split into segments and reassembled on the recipient's phone. You are typically billed per segment, so length affects cost. The SMS segment calculator counts the units in a finished message.
Is SMS secure enough for one-time codes?
SMS lacks end-to-end encryption. Codes can be intercepted or phished. You need to account for those risks before choosing SMS for an account or action.
Where Bird fits
You choose an eligible sender, confirm recipient permission and track delivery when sending SMS from your application. Bird’s SMS service connects those requests to carriers and reports their delivery receipts. RCS supports richer content through a separate business messaging route. Bird Verify manages code generation, delivery and channel selection for verification flows.